Microelectromechanical gyroscope with continuous self-test function, and method for controlling a microelectromechanical gyroscope

ABSTRACT

A microelectromechanical gyroscope includes a body and a sensing mass, which is movable with a degree of freedom in response to rotations of the body about an axis. A self-test actuator is capacitively coupled to the sensing mass for supplying a self-test signal. The capacitive coupling causes, in response to the self-test signal, electrostatic forces that are able to move the sensing mass in accordance with the degree of freedom at an actuation frequency. A sensing device detects transduction signals indicating displacements of the sensing mass in accordance with the degree of freedom. The sensing device is configured for discriminating, in the transduction signals, spectral components that are correlated to the actuation frequency and indicate the movement of the sensing mass as a result of the self-test signal.

BACKGROUND

1. Technical Field

The present invention relates to a microelectromechanical gyroscope withcontinuous self-test function, and to a method for controlling amicroelectromechanical gyroscope.

2. Description of the Related Art

As is known, the use of microelectromechanical systems (MEMS) hasincreasingly spread in various technological sectors and has yieldedencouraging results especially in providing inertial sensors,micro-integrated gyroscopes, and electromechanical oscillators for awide range of applications.

MEMS systems of this type are usually based upon microelectromechanicalstructures comprising at least one mass, connected to a fixed body(stator) through springs and movable with respect to the statoraccording to pre-set degrees of freedom. The movable mass and the statorare capacitively coupled through a plurality of respective comb-fingeredand mutually facing electrodes so as to form capacitors. The movement ofthe movable mass with respect to the stator, for example on account ofan external stress, modifies the capacitance of the capacitors, whenceit is possible to trace back to the relative displacement of the movablemass with respect to the fixed body and consequently to the forceapplied. Instead, by supplying appropriate biasing voltages, it ispossible to apply an electrostatic force to the movable mass for settingit in motion. In addition, for providing electromechanical oscillators,the frequency response of MEMS inertial structures is exploited, whichis typically of a second-order low-pass type, with one resonantfrequency.

In particular, MEMS gyroscopes have a more complex electromechanicalstructure, which comprises two masses that are movable with respect tothe stator and are coupled to one another so as to have a relativedegree of freedom. The two movable masses are both capacitively coupledto the stator. One of the masses is dedicated to driving and is kept inoscillation at the resonant frequency. The other mass is driven in(translational or rotational) oscillatory motion and, in the event ofrotation of the microstructure with respect to a pre-determinedgyroscopic axis with an angular velocity, is subject to a Coriolis forceproportional to the angular velocity itself. In practice, the drivenmass, which is capacitively coupled to the fixed body throughelectrodes, as likewise the driving mass, operates as an accelerometerthat enables detection of the Coriolis force and acceleration and henceof the angular velocity.

As practically any other device, MEMS gyroscopes are subject toproduction defects (which can regard the entire microstructure and theelectronics) and wear, which can reduce the reliability thereof orjeopardize their operation completely.

For this reason, prior to being installed, gyroscopes undergo testing inthe factory for proper operation, which enables identification andrejection of faulty examples.

In many cases, however, it is important to be able to carry out sampledchecks at any stage of the life of the gyroscope, after itsinstallation. In addition to the fact that, in general, it isadvantageous be able to locate components affected by faults, to proceedto their replacement, MEMS gyroscopes are used also in criticalapplications, where any malfunctioning may have disastrous consequences.Just to provide an example, in the automotive sector the activation ofmany air-bag systems is based upon the response provided by gyroscopes.It is thus evident how important it is to be able to equip MEMSgyroscopes with devices that are able to carry out frequent tests onproper operation.

It should moreover be noted that the circuits for testing do not have toaffect significantly the overall dimensions and the consumption levels,which assume increasing importance in a large number of applications.

In this connection, solutions have been developed that enable executionof in-field tests for proper operation in given conditions. According toknown solutions, in particular, a self-test signal is generated startingfrom driving signals used for keeping the driving mass in oscillatorymotion. The self-test signal is synchronous with the oscillations of thedriving mass and is supplied to electrodes for self-testing of thesensing mass, which are configured so as to apply electrostatic forcesto the sensing mass in the sensing direction, in response to theself-test signal. In practice, then, the self-test signal causes aneffect that is altogether similar to that of a rotation of themicrostructure about the gyroscopic axis. The amplitude of the self-testsignal is known. In conditions of absence of any rotation of themicrostructure, it is hence possible to detect the response of thegyroscope and, by comparing it with an expected response, determinewhether the gyroscope functions properly or whether any malfunctioninghas arisen.

BRIEF SUMMARY

It is noted that the known self-test process described above requiresthat the gyroscope be at rest, which is often impractical or evenimpossible, and that the self-test function cannot be exploitedcontinuously or simultaneously with normal operation.

According to one embodiment, a device is provided that includes amicroelectromechanical gyroscope having a body, a driving mass movablycoupled to the body, and a sensing mass movably coupled to the drivingmass. An actuator is provided, configured to apply a biasing force tothe sensing mass to introduce a corresponding motion to the sensing massrelative to the driving mass. A sensing device is configured to detectmotion of the sensing mass and to distinguish in the detected motion acomponent corresponding to motion of the body from a componentcorresponding to motion of the sensing mass introduced by the biasingforce applied by the actuator. The sensing device is capacitivelycoupled to the sensing body, and is configured to detect differentialchanges in the capacitive coupling arising in response to motion of thesensing mass and to produce a corresponding first signal.

The sensing device can include demodulators configured to extract, fromthe first signal, a second signal corresponding to the motion of thebody, and the second demodulator configured to extract, from the secondsignal, a third signal corresponding to motion of the sensing massintroduced by the biasing force applied by the actuator. Detection ofthe third signal at an appropriate amplitude is indicative of properfunction of the gyroscope.

According to an embodiment, the sensing mass is one of a plurality ofsensing masses, each movably coupled to the driving mass according to arespective axis of freedom, and the actuator is configured to apply abiasing force to each of the plurality of sensing masses to introduce acorresponding motion to each sensing mass according to the respectiveaxis of freedom. The sensing device is configured to detect motion ofeach of the sensing masses according to the respective axis of freedomand to distinguish in the detected motion a component corresponding tomotion of the body in a corresponding one of a plurality of axes ofdetection from a component corresponding to motion of the respectivesensing mass introduced by the biasing force applied by the actuator.

According to respective embodiments, the device is a palm-top computer,a laptop computer, a cell phone, a messaging device, a digital musicplayer, and a digital camera, each of which incorporates the gyroscopeto detect movement of the device.

According to an embodiment, a method for testing the operation of amicroelectromechanical gyroscope is provided, including introducing afirst signal to a sensing mass that is movably coupled to a body of thegyroscope, detecting a second signal corresponding to movement of thesensing mass according to a degree of freedom, and separating from thesecond signal a third signal corresponding to movement of the bodyaccording to a sensing axis of the gyroscope and a fourth signalcorresponding to movement of the sensing mass in response tointroduction of the first signal. The first signal can be introduced bymodulating a fifth signal having a frequency equal to a drive frequencyof the sensing mass with a sixth signal having a frequency lower thanthe drive frequency, to produce the first signal, and introducing theresulting first signal to the sensing mass.

Separating the third and fourth signals from the second signal caninclude demodulating the second signal with a seventh signal having afrequency equal to the drive frequency, to produce a first demodulatedsignal, and filtering the first demodulated signal to derive the thirdsignal; and demodulating the first demodulated signal with an eighthsignal having a frequency equal to the frequency of the sixth signal, toproduce a second demodulated signal, and filtering the seconddemodulated signal to derive the fourth signal.

Proper function of the gyroscope can be determined on the basis of anamplitude of the fourth signal, with respect to a reference value.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the invention, some embodiments thereofwill now be described, purely by way of non-limiting example and withreference to the attached drawings, wherein:

FIG. 1 is a simplified block diagram of a microelectromechanicalgyroscope in accordance with a first embodiment of the presentinvention;

FIG. 2 is a simplified top plan view of an enlarged detail of the deviceof FIG. 1;

FIG. 3 is a simplified top plan view of a further enlarged detail of thedevice of FIG. 1;

FIGS. 4 a-4 c are graphs regarding quantities used in the gyroscope ofFIG. 1;

FIG. 5 is a simplified block diagram of a microelectromechanicalgyroscope in accordance with a second embodiment of the presentinvention;

FIG. 6 shows time plots regarding parts of the gyroscope of FIG. 5 indifferent operating configurations; and

FIG. 7 is a simplified block diagram of an electronic systemincorporating a microelectronic device according to one embodiment ofthe present invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of a microelectromechanical gyroscope1, which comprises a microstructure 2, made of semiconductor material, adriving device 3, a self-test actuator 4, and a sensing device 5.

The microstructure 2 is made of semiconductor material and comprises afixed structure 6, a driving mass 7, and at least one sensing mass 8.For reasons of simplicity, in the embodiment illustrated hereinreference will be made to the case of a uniaxial gyroscope, in whichonly one sensing mass 8 is present. The ensuing description applies,however, also to the case of multiaxial gyroscopes, which comprise twoor more sensing masses for detecting rotations according to respectiveindependent axes.

The driving mass 7 is elastically connected by suspensions (not shown)to the fixed structure 6 so as to be oscillatable about a rest positionaccording to a translational or rotational degree of freedom.

The sensing mass 8 is mechanically coupled to the driving mass 7 so asto be driven in motion according to the degree of freedom of the drivingmass 7 itself. In addition, the sensing mass 8 is elastically connectedto the driving mass 7 so as to oscillate in turn with respect to thedriving mass 7 itself, with a respective further translational orrotational degree of freedom. In the embodiment described herein, inparticular, the driving mass 7 is linearly movable along a driving axisX, whilst the sensing mass 8 is movable with respect to the driving mass7 according to a sensing axis Y perpendicular to the driving axis X. Itis understood, however, that the type of movement (whether translationalor rotational) allowed by the degrees of freedom and the arrangement ofthe driving and sensing axes can vary according to the type ofgyroscope. With reference to the movements of the driving mass 7 and ofthe sensing mass 8, moreover, either of the expressions “according to anaxis” and “in accordance with an axis” can be understood as indicatingmovements along an axis or about an axis, according to whether themovements allowed to the masses by the respective degrees of freedom ofa particular device are translational or else rotational, respectively.Likewise, either of the expressions “according to a degree of freedom”and “in accordance with a degree of freedom” can be understood asindicating either translational or rotational movements, as allowed bythe degree of freedom itself.

In addition, the driving mass 7 (with the sensing mass 8) is connectedto the fixed structure 6 so as to define a resonant mechanical systemwith a resonant frequency ω_(R) (according to the driving axis X).

The driving mass 7 (FIG. 2) is capacitively coupled to the fixedstructure 6 by driving units 10 and feedback sensing units 12. Thecapacitive coupling is preferably of a differential type.

In greater detail, the driving units 10 comprise first and second fixeddriving electrodes 10 a, 10 b, anchored to the fixed structure 6 andextending substantially perpendicular to the driving direction X, andmovable driving electrodes 10 c, anchored to the driving mass 7 andarranged also substantially perpendicular to the driving direction X.The movable driving electrodes 10 c are comb-fingered and capacitivelycoupled to respective first fixed driving electrodes 10 a and secondfixed driving electrodes 10 b. In addition, the first and second fixeddriving electrodes 10 a, 10 b of the driving units 10 are electricallyconnected, respectively, to a first driving terminal 13 a and to asecond driving terminal 13 b of the microstructure 2. As alreadymentioned, moreover, the coupling is of a differential type. In otherwords, in each driving unit 10 a movement of the driving mass 7 alongthe driving axis X produces an increase in the capacitance between eachmovable driving electrode 10 c and one of the corresponding fixeddriving electrodes 10 a, 10 b, while the capacitance between therespective movable driving electrode 10 c and the other of thecorresponding fixed driving electrodes 10 a, 10 b decreases,accordingly.

The structure of the feedback sensing units 12 is similar to that of thedriving units 10. In particular, the feedback sensing units 12 comprisefirst and second fixed sensing electrodes 12 a, 12 b, anchored to thefixed structure 6, and first movable sensing electrodes 12 c, anchoredto the driving mass 7 and comb-fingered and capacitively coupled torespective first fixed sensing electrodes 12 a and second fixed sensingelectrodes 12 b. In addition, the first and second fixed sensingelectrodes 12 a, 12 b of the feedback sensing units 12 are electricallyconnected, respectively, to a first feedback sensing terminal 14 a andto a second feedback sensing terminal 14 b of the microstructure 2.

The sensing mass 8 is capacitively coupled to the fixed structure 6through signal sensing units 15 (FIG. 3) and self-test actuation units17. More precisely, the signal sensing units 15 comprise third andfourth fixed sensing electrodes 15 a, 15 b, anchored to the fixedstructure 6, and second movable sensing electrodes 15 c, anchored to thesensing mass 8 and set between respective third fixed sensing electrodes15 a and fourth fixed sensing electrodes 15 b. In addition, the thirdand fourth fixed sensing electrodes 15 a, 15 b of the signal sensingunits 15 are electrically connected, respectively, to a first signalsensing terminal 16 a and to a second signal sensing terminal 16 b ofthe microstructure 2.

The self-test actuation units 17 comprise first and second fixedself-test electrodes 17 a, 17 b, anchored to the fixed structure 6, andmovable self-test electrodes 17 c, anchored to the sensing mass 8 andset between respective first fixed sensing electrodes 17 a and secondfixed sensing electrodes 17 b. In addition, the first and second fixedself-test electrodes 17 a, 17 b of the self-test sensing units 17 areelectrically connected, respectively, to a first self-test terminal 18 aand to a second self-test terminal 18 b of the microstructure 2.

Also for the electrodes of the signal sensing units 15 and of theself-test actuation units 17 the capacitive coupling is of adifferential type, but is obtained by parallel-plate electrodes,perpendicular to the sensing direction Y.

As illustrated schematically in FIG. 1, in practice the driving mass 7is coupled to the driving terminals 13 a, 13 b through drivingdifferential capacitances C_(D1), C_(D2) and to the sensing terminals 14a, 14 b through feedback sensing differential capacitances C_(FBS1),C_(FBS2). The sensing mass 8 is instead coupled to the signal sensingterminals 17 a, 17 b through signal sensing differential capacitancesC_(SS1), C_(SS2), and to the self-test terminals 18 a, 18 b throughself-test differential capacitances C_(ST1), C_(ST2). In particular, theself-test actuation electrodes 17 a, 17 b are shaped in such a way thatthe sensing mass 8 is subject to an electrostatic actuation force F_(A)parallel to the sensing direction Y in the presence of a non-zerovoltage between the sensing mass 8 itself and the self-test terminals 18a, 18 b. In addition, the sensing mass 8 is connected to an excitationterminal 8 a for receiving a square-wave excitation signal V_(E),supplied for example by the driving device 3.

With reference once again to FIG. 1, the driving device 3 is connectedto the driving terminals 13 a, 13 b and to the feedback sensingterminals 14 a, 14 b of the microstructure 2 so as to form, with thedriving mass 7, a microelectromechanical loop 19. The driving device 3is configured so as to maintain the microelectromechanical loop 19 inoscillation at a driving frequency ω_(D) close to the resonant frequencyω_(R) of the mechanical system defined by the driving mass 7 (with thesensing mass 8) connected to the fixed structure 6. In addition, thedriving device 3 supplies a carrier signal V_(C) of a frequency equal tothe driving frequency ω_(D) and in phase with the oscillations of themicroelectromechanical loop 19.

The self-test actuator 4 comprises a frequency-divider module 20 and amodulator 21, having outputs connected to the self-test terminals 18 a,18 b of the microstructure 2. The self-test actuator 4 is thuscapacitively coupled to the sensing mass 8.

The frequency-divider module 20 is coupled to the driving device 3 forreceiving the carrier signal V_(C) and generates a base-band self-testsignal V_(STB), for example a sinusoidal or square-wave signal, with afrequency equal to or lower than the driving frequency ω_(D). Inparticular, the base-band self-test signal V_(STB) has a self-testfrequency ω_(ST) equal to ω_(D)/A (with A being an integer) and higherthan the frequency band of the signals indicating the Coriolisacceleration to which the sensing mass 8 is subject as a result of arotation about the sensing axis Y. Said frequency band will behereinafter referred to as Coriolis band BC. Preferably, the self-testfrequency ω_(ST) is equal to one half of the driving frequency ω_(D)(A=2) and is of the order of kilohertz (whilst the upper margin of theCoriolis band BC is of the order of tens of hertz).

The modulator 21 has inputs 21 a, 21 b, respectively connected to thefrequency-divider module 20 and to the driving device 3 for receivingthe carrier signal V_(C) and the base-band self-test signal V_(STB). Inaddition, the modulator 21 generates a modulated self-test signalV_(STM), which is in practice obtained by modulating the carrier signalV_(C) with the base-band self-test signal V_(STB). The modulation ispreferably of the DSB-SC (Double Side Band—Suppressed Carrier) type.

Outputs of the modulator 21 are coupled to the self-test terminals 18 a,18 b of the microstructure 2 so that the modulated self-test signalV_(STM) is applied to the self-test actuation units 17 and produces anet electrostatic actuation force F_(A) that tends to shift the sensingmass 7 according to the sensing axis Y (for example, in the directionindicated in FIG. 2).

The sensing device 5 comprises a read interface 23, a signal branch 24,and a self-test branch 25.

The read interface 23 comprises a charge-to-voltage converter, forexample a fully differential charge amplifier, having inputs connectedto the signal sensing terminals 16 a, 16 b of the microstructure 2. Theread interface 23 receives from the sensing terminals 16 a, 16 b of themicrostructure 2 electrical sensing signals indicating the displacementsof the sensing mass 8 according to the sensing axis Y. The electricalsensing signals can be charge packets Q_(S1), Q_(S2) having a valuemodulated by the angular velocity Ω_(C), as in the embodiment describedherein, or else currents or voltages.

The read interface 23 converts the electrical sensing signals (in theexample, the charge packets Q_(S1), Q_(S2)) into a transduction signalV_(T), in turn indicating the displacements of the sensing mass 8according to the sensing axis Y.

The signal branch 24 comprises a signal demodulator 27 and a firstlow-pass filter 28, which are cascade-connected to the read interface23. In greater detail, the signal demodulator 27 is coupled to outputsof the read interface 23 for receiving the transduction signal V_(T). Inaddition, the signal demodulator 27 has a demodulation input 27 aconnected to the driving device 3 for receiving the carrier signalV_(C). The transduction signal V_(T) is demodulated using the carriersignal V_(C). A first demodulated signal V_(D1) is thus present on theoutput of the signal demodulator 27.

The first low-pass filter 28 is connected downstream of the signaldemodulator 27 and has a cut-off frequency comprised betweenapproximately the upper margin of the Coriolis band BC and the self-testfrequency ω_(ST). Preferably, the cut-off frequency is close to theupper margin of the Coriolis band BC so as not to jeopardize the signalcomponents produced by the rotation about the sensing axis Y.

The output of the first low-pass filter 28 defines a signal output 1 aof the gyroscope 1 and supplies an angular-velocity signal V_(Ω)indicating the angular velocity Ω about the sensing axis Y.

The self-test branch 25 comprises a self-test demodulator 30, a secondlow-pass filter 31, a comparator 32, and a status register 33,cascade-connected together.

In detail, the self-test demodulator 30 is coupled to the signaldemodulator 27, for receiving the first demodulated signal V_(D1), andhas a demodulation input 30 a connected to the frequency-divider module20 of the self-test actuator 4, for receiving the base-band self-testsignal V_(STB). The self-test demodulator 30, in practice, carries out afurther demodulation of the first demodulated signal V_(D1), using thebase-band self-test signal V_(STB) as carrier, and thus generates asecond demodulated signal V_(D2).

The second low-pass filter 31 is connected downstream of the self-testdemodulator 30 and has a cut-off frequency such as to attenuate theharmonic components of the second demodulated signal V_(D2) equal to orhigher than the self-test frequency ω_(ST). In one embodiment, the firstlow-pass filter 28 and the second low-pass filter 31 have the samecut-off frequency. However, the second low-pass filter 31 has the basicpurpose of preserving the DC component of the second demodulated signalV_(D2) and attenuating or eliminating all the higher components. Thus,the cut-off frequency of the second low-pass filter 31 may even be lowerthan that of the first low-pass filter 28. A DC self-test signal V_(STC)is present on the output of the second low-pass filter 31.

The comparator 32 is coupled to the low-pass filter 31 for receiving theDC self-test signal V_(STC,) and to a programmable reference generator35, which provides a threshold value V_(TH). The comparator 32 suppliesa self-test logic signal FST having a first value when the DC self-testsignal V_(STC) is greater than the threshold value V_(TH), and a secondlogic value otherwise.

The status register 33 stores the self-test logic signal FST and makesit available as status flag. In particular, the first logic value of theself-test logic signal FST indicates proper operation of the gyroscope1, whilst the second logic value indicates malfunctioning.

Operation of the gyroscope 1 is described in what follows.

The driving device 3 acts so as to maintain the microelectromechanicalloop 19 in oscillation at the driving frequency ω_(D). Consequently, thedriving mass 7 and the sensing mass 8 vibrate along the driving axis Xabout a resting position. When the gyroscope 1 turns about a gyroscopicaxis perpendicular to the driving axis X and to the sensing axis Y, thesensing mass 8 oscillates along the sensing axis Y as a result of theCoriolis force. For reasons of simplicity, we shall assume that thegyroscope turns with a constant angular velocity Ω_(C) in the Coriolisband BC (without any loss of generality to the ensuing treatment, whichapplies also in the case of angular velocity variable within theCoriolis band BC). The amplitude of the displacement, which isproportional to the rotation rate about the gyroscopic axis and to thevelocity along the driving axis X, is transduced by the read interface23 and converted into a spectral component of the transduction signalV_(T). More precisely, from the standpoint of the harmonic content ofthe transduction signal V_(T) the effect of the rotation about thegyroscopic axis is that of a carrier signal with driving frequency ω_(D)amplitude-modulated by a signal having a frequency equal to the angularvelocity Ω_(C). As illustrated in FIG. 4 a, then, the component of thetransduction signal V_(T) due to rotation about the gyroscopic axiscomprises the frequencies ω_(D)±Ω_(C) (more in general, the bandω_(D)±BC in the case of rotations at variable angular velocity,illustrated here with a dashed line).

Simultaneously, the self-test actuator 4 applies the modulated self-testsignal V_(STM) to the self-test electrodes 18 a, 18 b of themicrostructure 2. Thanks to the capacitive coupling with the sensingmass 8, the modulated self-test signal V_(STM) produces an electrostaticactuation force F_(A) and consequently causes displacements of thesensing mass 8 along the axis Y with actuation frequencies ω_(D)±ω_(ST).In normal operating conditions, said displacements are proportional tothe amplitude of the modulated self-test signal V_(STM); in effect, theyadd to the displacements caused by the rotation about the gyroscopicaxis and, like these, are transduced by the read interface 23.Consequently, the transduction signal V_(T) also comprises a componentindicating the displacements caused by the self-test actuator 4, eventhough in a different frequency band with respect to the component dueto rotation about the gyroscopic axis. In particular, given that themodulated self-test signal V_(STM) is obtained by DSB-SC modulation ofthe carrier signal V_(C) with the base-band self-test signal V_(STB),its spectrum contains the actuation frequencies ω_(D)±ω_(ST), which areagain encountered also in the spectrum of the transduction signal V_(T).To sum up, the sensing mass 8 operates as adder of the effects due tothe rotation about the gyroscopic axis (which it is desired to measurethrough the gyroscope 1) and to the modulated self-test signal V_(STM)supplied by the self-test actuator 4, and the transduction signal V_(T)generated by the read interface 23 contains (but for disturbance) thespectral components ω_(D)±ω_(ST) (actuation frequencies) and ω_(D)±Ω_(C)(ω_(D)±BC).

The demodulation performed by the signal demodulator 27 brings thefrequency components ω_(ST) and Ω_(C) back into band base, in addition,of course, to introducing frequency components 2ω_(D)±ω_(ST) and2ω_(D)±ω_(C) (FIG. 4 b; in practice, the spectrum of the firstdemodulated signal V_(D1) contains the frequencies ω_(D)±(ω_(D)±ω_(ST))and ω_(D)±Ω_(C), or ω_(D)±(ω_(D)±BC) in the case of rotations atvariable angular velocity).

The first low-pass filter 28 (the transfer function FLP1 of which isillustrated in FIG. 4 b) eliminates or in any case attenuates all thespectral components having a frequency higher than the Coriolis band BC,and hence the angular-velocity signal V_(Ω) indicates just the angularvelocity Ω_(C) about the gyroscopic axis.

The self-test demodulator 30 carries out a further demodulation of thefirst demodulated signal V_(D1) using the base-band self-test signalV_(STB) as carrier. The component of the first demodulated signal V_(D1)at the self-test frequency ω_(ST) originates a DC component DC_(ST)(FIG. 4 c), which is due exclusively to the effect of the modulatedself-test signal V_(STM) on the movement of the sensing mass 8 andcorresponds (is for example proportional) to the maximum amplitude ofthe modulated self-test signal V_(STM) itself The other components ofthe first demodulated signal V_(D1) introduce higher frequencycomponents into the spectrum of the second demodulated signal V_(D2),among which frequency components ω_(ST)±Ω_(C) (further spectralcomponents are not illustrated for reasons of simplicity).

The second low-pass filter 31 (the transfer function FLP2 of which isillustrated in FIG. 4 c) eliminates or attenuates all the componentswith frequency higher than the DC component. The DC self-test signalV_(STC) is hence indicative only of the contribution of the modulatedself-test signal V_(STM) and can thus be compared with a threshold valueV_(TH), which can possibly be calibrated. As already mentioned, inpractice, the effect of the base-band self-test signal V_(STB) isequivalent, as regards the displacements of the sensing mass 8, to arotation about the gyroscopic axis, but falls within a differentfrequency band, which the self-test branch 25 makes it possible toisolate and discriminate in order to determine whether the gyroscope 1is operating properly. If in fact any malfunctioning arises, whether inthe micromechanical part or in the associated electronics, the amplitudeof the DC self-test signal V_(STC) is reduced with respect to theexpected value. The comparator 32 then switches, and the self-test logicsignal FST passes to the value indicating a fault, which is immediatelyidentified.

In practice, then, the gyroscope 1 uses a self-test signal (themodulated self-test signal V_(STM)) with harmonic content basically in aband distinct from the Coriolis band BC. The effect of the self-testsignal can be added to the effect of rotations about the gyroscopic axisthrough the sensing mass 8. Hence, the signals fetched and transduced bythe read interface 23 have spectral components in distinct bands thatcan be put down either to the rotations about the gyroscopic axis or tothe self-test actuation. The reading branch 24 and the self-test branch25 of the reading device 5 separate the spectral components, inparticular isolating the contribution of the self-test actuation. Thelatter can hence be examined for determining the compatibility withnormal operating conditions. In the embodiment described, the separationof the spectral components is obtained effectively by a doubledemodulation, using first the carrier signal V_(C) and then thebase-band self-test signal V_(ST) as demodulating signals.

The gyroscope 1 described above advantageously enables continuousexploitation of the self-test functions, irrespective of the operatingconditions. In particular, the self-test can be conducted even when thegyroscope 1 is not at rest, and it is not necessary to suspend theoperations of measurement of the angular velocity, because the self-testactuator 4 and the self-test branch 25 of the sensing device 5 enablediscrimination, in the spectrum of the signal transduced by the readinterface 23, of the effect of the self-test signal applied to thesensing mass 8. In addition, using a very selective low-pass filter forisolating the DC component, it is possible to apply self-test signals ofreduced amplitude, which practically have no effect as regards sensingof the angular velocity (in other words, the dynamics of the readinterface 23 is almost entirely available for detection of theangular-velocity signal, and the self-test signals have an altogethernegligible effect on the signal-to-noise ratio).

FIG. 5 illustrates a triaxial gyroscope 100 in accordance with adifferent embodiment of the invention. The gyroscope 100 comprises amicrostructure 102, a driving device 103, a self-test actuator 104, asensing device 105, and a control unit 150.

The microstructure 102 is, for example, of the type described in detailin the published European patent application No. EP-A-1 832 841 and inthe corresponding published U.S. patent application No. US 2007/0214881A1 and comprises a fixed structure 106, a driving mass 107, and threesensing-mass systems 108-X, 108-Y, 108-Z. In FIG. 5, however, themicrostructure 102 is represented only schematically, for reasons ofsimplicity.

The driving mass 107 is elastically connected through suspensions (notshown) to the fixed structure 106 so as to oscillate in a plane XY abouta resting position according to a degree of freedom, in this caserotational. The sensing masses 108-X, 108-Y, 108-Z are mechanicallycoupled to the driving mass 107 so as to be driven in motion accordingto the rotational degree of freedom of the driving mass 107 itself. Inaddition, the sensing masses 108-X, 108-Y, 108-Z are elasticallyconnected to the driving mass 107 so as to oscillate in turn withrespect to the driving mass 107 itself, with respective furthertranslational or rotational degrees of freedom, in response to rotationsof the microstructure 102 about respective mutually perpendicularsensing axes X, Y, Z.

The sensing masses 108-X, 108-Y, 108-Z are capacitively differentiallycoupled to the fixed structure 106 through respective sets of sensingelectrodes 115-X, 115-Y, 115-Z (here not shown individually andillustrated schematically as capacitors), which are connected torespective pairs of signal sensing terminals 116-X, 116-Y, 116-Z. Thesets of sensing electrodes 115-X, 115-Y, 115-Z are shaped in such a waythat, in the presence of an electrical signal on the signal sensingterminals 116-X, 116-Y, 116- Z, the corresponding sensing masses 108-X,108-Y, 108-Z are subject to electrostatic forces according to therespective degrees of freedom.

The driving device 103 is connected to the microstructure 102 so as toform, with the driving mass 107, a microelectromechanical loop 119. Thedriving device 103 is configured so as to maintain themicroelectromechanical loop 119 in oscillation at a driving frequencyω_(D) close to the resonant frequency ω_(R) of the mechanical systemdefined by the driving mass 107 (with the sensing mass 108) connected tothe fixed structure 106. In addition, the driving device 103 supplies acarrier signal V_(C) with a frequency equal to the driving frequencyω_(D) and in phase with the oscillations of the microelectromechanicalloop 119.

The self-test actuator 104 comprises a frequency divider 120 and amodulator 121, substantially as already described with reference to FIG.1, and, moreover, a first demultiplexer 122, which is controlled by afirst selection signal SEL1 generated by a control unit 150. Inparticular, the frequency-divider module 120 generates a base-bandself-test signal V_(STB), for example a sinusoidal or square-wavesignal, with a frequency equal to or lower than the driving frequencyω_(D), starting from the carrier signal V_(C). The modulator 121generates a modulated self-test signal V_(STM) that is obtained bymodulating the carrier signal V_(C) with the band-base self-test signalV_(STB). In this case, the modulated self-test signal V_(STM) issupplied on outputs of the modulator 121 that are selectivelyconnectable, through the first demultiplexer 122, to the signal sensingterminals 116-X, 116-Y, 116-Z of the microstructure 102. The firstdemultiplexer 122 is moreover configured so as to decouple in acontrolled way the modulator 121 from the signal sensing terminals116-X, 116-Y, 116-Z during sensing steps of each read cycle of thegyroscope 100 while the sensing circuit 105 is connected to the signalsensing terminals 116-X, 116-Y, 116-Z. For example, the modulator 121may be provided with a floating output or else an enable output thatenables setting of the outputs in a high-impedance state.

The sensing device 105 comprises a multiplexer 126, a read interface123, a signal demodulator 127, a self-test demodulator 130, a low-passfilter 131, a comparator 132, a second demultiplexer 134, and a statusregister 133.

The read interface 123 is selectively connectable in succession to thesignal sensing terminals 116-X, 116-Y, 116-Z through the multiplexer126, which is controlled by a second selection signal SEL2. In addition,the multiplexer 126 is configured so as to decouple the read interface123 from the signal sensing terminals 116-X, 116-Y, 116-Z through themultiplexer 126 during self-test steps of each read cycle of thegyroscope 100, where the signal sensing terminals 116-X, 116-Y, 116-Zreceive the modulated self-test signal V_(STM). When connected, the readinterface 23 receives from the signal sensing terminals 116-X, 116-Y,116-Z electrical sensing signals (charge packets in the embodimentdescribed) and converts them into respective transduction signalsV_(TX), V_(TY), V_(TZ).

The signal demodulator 127 receives in cyclic succession thetransduction signals V_(TX), V_(TY), V_(TZ) from the read interface 123.In addition, a demodulation input 127 a of the signal demodulator 127 isconnected to the driving device 3 for receiving the carrier signalV_(C). The transduction signals V_(TX), V_(TY), V_(TZ) are demodulatedusing the carrier signal V_(C). On the output of the signal demodulator27 there are hence cyclically present first demodulated signals V_(D1X),V_(D1Y), V_(D1Z) (in other words, the signal present on the output ofthe signal demodulator 127 cyclically represents the movements of thesensing masses 108-X, 108-Y, 108, which are all due to rotations aboutthe axes X, Y, Z and to self-test actuation).

The self-test demodulator 130 is coupled to the signal demodulator 127,for receiving the first demodulated signals V_(D1X), V_(D1Y), V_(D1Z)cyclically and has a demodulation input 130 a connected to thefrequency-divider module 120 for receiving the base-band self-testsignal V_(STB). The self-test demodulator 130 carries out a furtherdemodulation of the first demodulated signals V_(D1X), V_(D1Y), V_(D1Z)using the base-band self-test signal V_(STB) as carrier and thusgenerates cyclically second demodulated signals V_(D2X), V_(D2Y),V_(D2Z).

The second demultiplexer 134, which is controlled by a third selectionsignal SEL3, receives the first demodulated signals V_(D1X), V_(D1Y),V_(D1Z) from the signal demodulator 127 and forwards them on respectivesignal output lines 134-X, 134-Y, 134-Z. In addition, the seconddemultiplexer 134 receives the second demodulated signals V_(D2X),V_(D2Y), V_(D2Z) from the self-test demodulator 130 and forwards them ona self-test output line 134-ST.

The low-pass filter 131 has a plurality of filtering lines, connected toa respective one between the signal output lines 134-X, 134-Y, 134-Z andthe self-test output line 134-ST. The cut-off frequency of the low-passfilter 131 is comprised between approximately the upper margin of theCoriolis band BC and the self-test frequency ω_(ST). Signal outputs ofthe low-pass filter 131 (in particular, the outputs corresponding to thesignal output lines 134-X, 134-Y, 134-Z of the second demultiplexer 134)define respective signal outputs 100-X, 100- Y, 100-Z of the gyroscope100 and supply angular-velocity signals V_(ΩX), V_(ΩY), V_(ΩZ) thatindicate angular velocities Ω_(X), Ω_(Y), Ω_(Z) about the sensing axesX, Y, X, respectively.

A self-test output 131-ST of the low-pass filter 131, corresponding tothe self-test output line 134-ST of the second demultiplexer 134,cyclically supplies DC self-test signals V_(STCX), V_(STCY), V_(STCZ).

The comparator 131 is coupled to the self-test output 131-ST of thelow-pass filter 131, for receiving the DC self-test signals V_(STCX),V_(STCY), V_(STCZ), and to a programmable reference generator 135, whichyields a threshold value V_(TH). The comparator 131 supplies self-testlogic signals FST_(X), FST_(Y), FST_(Z), having a first value, when thecorresponding DC self-test signal V_(STCX), V_(STCY), V_(STCZ) is higherthan the threshold value V_(TH), and a second logic value otherwise. Theself-test logic signals FST_(X), FST_(Y), FST_(Z) are indicative ofproper operation of the gyroscope 100 each as regards a respective oneof the sensing axes X, Y, Z. In one embodiment (not illustrated),distinct threshold values are used for the different sensing axes.

The self-test logic signals FST_(X), FST_(Y), FST_(Z) may be madeavailable as status flags in a register (not shown).

The working principle of the gyroscope 100 is similar to what hasalready been described for the gyroscope 1, with the difference that, inthe gyroscope 100, the self-testing function is provided on each sensingaxis X, Y, Z using the time-division signal sensing terminals.

In practice, each read cycle T_(R) (FIG. 6) of the gyroscope 100 isdivided into three sensing intervals T_(X), T_(Y), T_(Z), each dedicatedto reading according to a respective one of the sensing axes X, Y, Z.The sensing intervals T_(X), T_(Y), T_(Z) each comprise a sensing stepT_(SX), T_(SY), T_(SZ) and a self-test step T_(STX), T_(STY), T_(STZ).In the sensing steps T_(SX), T_(SY), T_(SZ) dedicated to reading of therespective sensing axis X, Y, Z, the signal sensing terminals 116-X,116-Y, 116-Z of the sensing-mass systems 108-X, 108-Y, 108-Z areselectively coupled to the sensing device 105 through the multiplexer126 (not shown in FIG. 6). In each self-test step T_(STZ), T_(STY),T_(STZ), instead, the modulated self-test signal V_(STM) is applied tothe signal sensing terminals 116-X, 116-Y, 116-Z involved through thefirst demultiplexer 122 (not shown in FIG. 6).

As has been mentioned previously, the modulated self-test signal V_(STM)is generated by the modulation of the carrier signal V_(C) with thebase-band self-test signal V_(ST). Each sensing-mass system 108-X,108-Y, 108-Z sums up the contribution due to displacements caused by theself-test device 104 and the contribution due to rotation of thegyroscope 100 about the respective sensing axis X, Y, Z. The latterpresent as carrier signals with frequency ω_(D) modulated by angularvelocities Ω_(X), Ω_(Y), Ω_(Z), respectively, about the sensing axes X,Y, Z. The transduction signals V_(TX), V_(TY), V_(TZ) supplied insequence by the read interface 123 hence contain the spectral componentsΩ_(D)±ω_(ST) (actuation frequencies, due to the movements of the sensingmasses 108-X, 108- Y, 108-Z as a result of the modulated self-testsignal V_(STM)) and ω_(D)±BC (due to rotation of the gyroscope 100). Thesignal demodulator 127, as already described, introduces into basebandthe frequency components ω_(ST) and Ω_(C), whereas the self-testdemodulator 130 introduces a DC component that is due exclusively to theeffect of the self-test actuation on the movement of the sensing-masssystems 108-X, 108-Y, 108-Z and corresponds to the maximum amplitude ofthe modulated self-test signal V_(STM).

The low-pass filter 131 eliminates the components with frequency higherthan the self-test frequency ω_(D) so that at output theangular-velocity signals V_(ΩX), V_(ΩY), V_(ΩZ) are defined only by thecomponents in the Coriolis band BC, and the DC self-test signalsV_(STCX), V_(STCY), V_(STCZ) have substantially only DC components.

Finally, the DC self-test signals V_(STCX), V_(STCY), V_(STCZ) arecompared with respective threshold values by the comparator 131 togenerate the self-test logic signals FST_(X), FST_(Y), FST_(Z).

In addition to the advantages already referred to, regarding thepossibility of performing self-testing continuously and in any conditionof operation of the gyroscope, both the microstructure and the sensingdevice are simplified. In fact, the microstructure 102 does not requirededicated self-test terminals and is hence simpler to design and buildand is less subject to possible failures. The sensing device 105 isinstead provided with just one low-pass filter.

According to the embodiment of FIG. 6, the operation of the gyroscope100 is divided into three sensing intervals T_(X), T_(Y), T_(Z), eachdedicated to operation of a respective one of the sensing masses 108-X,108- Y, 108-Z, and each divided into a respective sensing step andself-test step. According to an alternative embodiment, the sensing stepT_(S) corresponding to each of the sensing masses overlaps, at leastpartially, with the self-test step T_(ST) of another of the sensingmasses. Illustrated in FIG. 7 is a portion of an electronic system 200in accordance with one embodiment of the present invention. The system200 incorporates the gyroscope 1 and may be used in devices, such as,for example, a palm-top computer (personal digital assistant, PDA), alaptop or portable computer, possibly with wireless capacity, a cellphone, a messaging device, a digital music player, a digital camera, orother devices designed to process, store, transmit or receiveinformation. For example, the gyroscope 1 may be used in a digitalcamera for detecting movements and performing an image stabilization. Inother embodiments, the gyroscope 1 is included in a portable computer, aPDA, or a cell phone for detecting a free-fall condition and activatinga safety configuration. In a further embodiment, the gyroscope 1 isincluded in a motion-activated user interface for computers orvideo-game consoles. In a further embodiment, the gyroscope 1 isincorporated in a satellite-navigation device and is used for temporaryposition tracking in the event of loss of the satellite-positioningsignal.

The electronic system 200 can comprise a controller 210, an input/output(I/O) device 220 (for example, a keyboard or a display), the gyroscope1, a wireless interface 240, and a memory 260, of a volatile ornonvolatile type, coupled to one another through a bus 250. In oneembodiment, a battery 280 may be used for supplying the system 200. Itshould be noted that the scope of the present invention is not limitedto embodiments having necessarily one or all of the devices listed.

The controller 210 can comprise, for example, one or moremicroprocessors, microcontrollers and the like.

The I/O device 220 may be used for generating a message. The system 200can use the wireless interface 240 for transmitting and receivingmessages to and from a wireless communications network with aradiofrequency signal (RF). Examples of wireless interface can comprisean antenna, a wireless transceiver, such as a dipole antenna, eventhough the scope of the present invention is not limited from thisstandpoint. In addition, the I/O device 220 can supply a voltagerepresenting what is stored either in the form of digital output (ifdigital information has been stored) or in the form of analoginformation (if analog information has been stored).

Finally, it is evident that modifications and variations may be made tothe method and to the device described, without departing from the scopeof the present invention, as defined in the annexed claims.

In the first place, the discrimination in frequency of the self-testsignals may be used for testing proper functionality of gyroscopes withany microelectromechanical structure that are based upon detection ofCoriolis forces.

The discrimination of the spectral component due to the self-testactuation may be conducted directly by the signals coming from thesignal demodulator or even directly by the read interface. In this case,a selective pass-band filter may be used, centered, for example, aroundthe frequency 2ω_(D)−ω_(ST) for the signals coming from the signaldemodulator, or else around the frequency ω_(D)−ω_(ST) for the signalscoming from the read interface.

Furthermore, the embodiments described above can be combined to providefurther embodiments. For example, according to an embodiment, in amicroelectromechanical device configured to detect rotation about asingle sensing axis, similar to the gyroscope 1 described with referenceto FIGS. 1-4, a modulator and a read interface can be cyclically coupledto a sensing mass via a multiplexer and a single capacitive coupling, asdescribed with reference to the gyroscope 100 of FIGS. 5 and 6.According to another embodiment, in a multi-axial gyroscope, such as,e.g., the device 100, each sensing mass can be provided with arespective self test actuator and a respective sensing device,substantially as described with reference to the device 1.

According to an embodiment, the electronic system 200 described withreference to FIG. 7 can be provided with a multi-axial device in placeof the gyroscope 1. As used herein, the symbol “±” means “plus andminus.” Thus, for example, the phrase “the frequencies ω_(D)±ω_(ST)” canbe interpreted as referring inclusively to the frequency ω_(D)+ω_(ST)and the frequency ω_(D)−ω_(ST).

All of the U.S. patents, U.S. patent application publications, U.S.patent application, foreign patents, foreign patent application andnon-patent publications referred to in this specification areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, application and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A microelectromechanical gyroscope comprising: a body; a sensingmass, elastically connected to the body and movable with respect to thebody according to a degree of freedom in response to rotations of thebody about a sensing axis; a self-test actuator that is connectable tothe sensing mass through a capacitive coupling, the capacitive couplingbeing configured to apply, in response to a self-test signal from theself-test actuator, electrostatic forces to move the sensing mass inaccordance with the degree of freedom and at an actuation frequency; anda sensing device configured to sense transduction signals indicative ofdisplacements of the sensing mass according to the degree of freedom,and to discriminate, in the transduction signals, spectral componentscorresponding to the actuation frequency and spectral componentsindicative of motion of the sensing mass caused by the self-test signal.2. The gyroscope according to claim 1, wherein the self-test actuatorcomprises a modulator having a first input configured to receive abase-band signal, and a second input configured to receive a carriersignal, and wherein the modulator is configured generate the self-testsignal by modulation of the carrier signal with the base-band signal. 3.The gyroscope according to claim 2, wherein the sensing device comprisesa signal demodulator having a demodulation input configured to receivethe carrier signal, and a transduction signal input coupled to thesensing mass and configured to receive the transduction signals, thesignal demodulator being configured to generate first demodulatedsignals by demodulation of the transduction signals with the carriersignal.
 4. The gyroscope according to claim 3, wherein the sensingdevice comprises a self-test demodulator having a signal input coupledto the signal demodulator and configured to receive the firstdemodulated signals, and a demodulation input configured to receive thebase-band signal the self-test demodulator being configured to generatesecond demodulated signals by demodulation of the first demodulatedsignals with the base-band signal.
 5. The gyroscope according to claim2, comprising: a driving mass, elastically coupled to the body so as tobe movable according to a driving degree of freedom, the sensing massbeing elastically coupled to the driving mass so as to be drawn inmotion by the driving mass in accordance with the driving degree offreedom and so as to be movable relative to the driving mass inaccordance with the degree of freedom; and a driving device, coupled tothe driving mass so as to form a microelectromechanical loop includingthe driving mass and configured to maintain the driving mass inoscillation in accordance to the driving degree of freedom and at adriving frequency.
 6. The gyroscope according to claim 5, wherein thedriving device is configured to provide the carrier signal and whereinthe carrier signal is at the driving frequency.
 7. The gyroscopeaccording to claim 6, wherein the base-band signal has a self-testfrequency equal to an integer submultiple of the driving frequency. 8.The gyroscope according to claim 7, wherein the self-test actuatorcomprises a frequency divider coupled to the driving device forreceiving the carrier signal and configured to provide the base-bandsignal.
 9. The gyroscope according to claim 6, wherein the sensingdevice comprises a low-pass filter, cascade-connected to the self-testdemodulator and having a cut-off frequency not greater than theself-test frequency.
 10. The gyroscope according to claim 1, wherein thecapacitive coupling comprises dedicated self-test electrodes, connectedto the self-test actuator for providing a self-test signal.
 11. Thegyroscope according to claim 1, wherein the capacitive couplingcomprises sensing electrodes and selective-connection devices controlledto connect the sensing electrodes selectively to the sensing device andto the self-test actuator in time division.
 12. The gyroscope accordingto claim 11, comprising a plurality of systems of sensing masses,elastically connected to the body and movable with respect to the bodyaccording to respective degrees of freedom, in response to rotations ofthe body about respective axes; wherein the self-test actuator isselectively connectable to each system of sensing masses throughrespective capacitive couplings by the selective-connection devices , toprovide the self-test signal; and wherein the capacitive couplings areconfigured to apply, in response to the self-test signal, electrostaticforces to move the systems of sensing masses according to the respectivedegrees of freedom and at the actuation frequency.
 13. A self-testmethod comprising: moving a sensing mass of a microelectromechanicalgyroscope in accordance with a degree of freedom and at an actuationfrequency, the moving including applying electrostatic forces to thesensing mass; sensing transduction signals indicative of displacementsof the sensing mass in accordance with a degree of freedom; anddiscriminating, in the transduction signals, spectral componentscorresponding to the actuation frequency and indicative of the motion ofthe sensing mass in response to the electrostatic forces.
 14. The methodaccording to claim 13, wherein applying electrostatic forces comprisessupplying a self-test signal to the sensing mass through a capacitivecoupling.
 15. The method according to claim 13, wherein supplying aself-test signal comprises modulating a carrier signal with a self-testsignal.
 16. The method according to claim 15, wherein discriminatingcomprises demodulating the transduction signals using the carriersignal, to generate a first demodulated signal.
 17. The method accordingto claim 16, wherein discriminating comprises further demodulating thefirst demodulated signal using the self-test signal, to generate asecond demodulated signal.
 18. A method, comprising: introducing a firstsignal to a sensing mass that is movably coupled to a body in amicroelectromechanical gyroscope; detecting a second signalcorresponding to movement of the sensing mass according to a degree offreedom; and separating from the second signal a third signalcorresponding to movement of the body according to a first axis and afourth signal corresponding to movement of the sensing mass in responseto introduction of the first signal.
 19. The method of claim 18 whereinintroducing a first signal comprises modulating a fifth signal having afrequency equal to a drive frequency of the sensing mass with a sixthsignal having a frequency lower than the drive frequency, to produce thefirst signal, and introducing the resulting first signal to the sensingmass.
 20. The method of claim 19 wherein separating from the secondsignal a third signal and a fourth signal comprises: demodulating thesecond signal with a seventh signal having a frequency equal to thedrive frequency, to produce a first demodulated signal; deriving thethird signal from the first demodulated signal; demodulating the firstdemodulated signal with an eighth signal having a frequency equal to thefrequency of the sixth signal, to produce a second demodulated signal;and deriving the fourth signal from the second demodulated signal. 21.The method of claim 20 wherein deriving the fourth signal comprisesextracting a DC component of the second demodulated signal.
 22. Themethod of claim 18, comprising determining, based on an amplitude of thefourth signal, whether the microelectromechanical gyroscope isfunctioning correctly.
 23. The method of claim 18 wherein: introducing afirst signal to a sensing mass comprises introducing the first signal insequence to each of a plurality of sensing masses that are movablycoupled to the body; detecting a second signal corresponding to movementof the sensing mass comprises detecting a second signal from each of theplurality of sensing masses corresponding to movement of the respectivesensing mass according to a respective degree of freedom; and separatingfrom the second signal a third signal and a fourth signal comprisesseparating from the second signal of each of the plurality of sensingmasses a respective third signal corresponding to movement of the bodyaccording to a respective one of a plurality of axes and a respectivefourth signal corresponding to movement of the respective sensing massin response to introduction of the first signal.
 24. A device,comprising: a microelectromechanical gyroscope having a body, a drivingmass movably coupled to the body, and a sensing mass movably coupled tothe driving mass; an actuator configured to apply a biasing force to thesensing mass to introduce a corresponding motion to the sensing massrelative to the driving mass; a sensing device configured to detectmotion of the sensing mass according to a degree of freedom thereof andto distinguish in the detected motion a component corresponding tomotion of the body from a component corresponding to motion of thesensing mass introduced by the biasing force applied by the actuator.25. The device of claim 24 wherein the actuator is capacitively coupledto the sensing body and the biasing force is produced by electrostaticoperation.
 26. The device of claim 24 wherein the sensing device iscapacitively coupled to the sensing body, and is configured to detectdifferential changes in the capacitive coupling arising in response tomotion of the sensing mass and to produce a corresponding first signal.27. The device of claim 26 wherein the sensing device comprises firstand second demodulators, the first demodulator configured to extract,from a first signal produced by the detected motion of the sensing mass,a second signal corresponding to the motion of the body, and the seconddemodulator configured to extract, from the second signal, a thirdsignal corresponding to motion of the sensing mass introduced by thebiasing force applied by the actuator.
 28. The device of claim 27wherein the actuator is configured to apply a modulated self-test signalto the sensing mass as the biasing force.
 29. The device of claim 24wherein: the sensing mass is one of a plurality of sensing masses, eachmovably coupled to the driving mass according to a respective axis offreedom; the actuator is configured to apply a biasing force to each ofthe plurality of sensing masses to introduce a corresponding motion toeach sensing mass according to the respective axis of freedom; thesensing device is configured to detect motion of each of the sensingmasses according to the respective axis of freedom and to distinguish inthe detected motion a component corresponding to motion of the body in acorresponding one of a plurality of axes of detection from a componentcorresponding to motion of the respective sensing mass introduced by thebiasing force applied by the actuator.
 30. The device of claim 29wherein: the actuator is configured to apply the biasing force to eachof the plurality of sensing masses sequentially; and the sensing deviceis configured to detect motion of each of the sensing massessequentially.
 31. The device of claim 30 wherein each of the sensingmasses comprises a respective single capacitive coupling, and whereinthe actuator and sensing device are configured to be capacitivelycoupled to each sensing mass alternately, via the respective singlecapacitive coupling.
 32. The device of claim 24 wherein the device isone of a palm-top computer, a laptop computer, a cell phone, a messagingdevice, a digital music player, or a digital camera, and wherein themicroelectromechanical gyroscope is incorporated in the device to detectmovement of the device.